75-80lb!! That's an insane difference. It would be very difficult to overcome, even for experienced clinchers. Try to clinch with someone 80lb lighter than you, and you'll get the idea. Being totally exhausted is normal.
Also, who says drilling clinch is pointless? That makes no sense. For reference, see how the Thais drill clinch in the library. See also how Judokas train foot sweeps, such as Deashi-harai.
If you can't find someone your size, you can still think of flow-based (instead of strength-based) drills to do together: "swimming," unbalancing, sweeps from 50/50, etc. You don't have to go full force all the time: you are learning to recognize certain common positions, their counters, counters to the counters, and smooth transitions between these.
The mismatch lb situation is common in BJJ training too. Ask your partner to go technical and not to rely on strength so much in your sessions.